1. Field of the Invention
The invention relates to the detection device for detecting a tuning state of a high-frequency modulated signal within a frequency band, having an output for supplying an in-channel signal indicating whether a desired tuning state has been reached. The detection device comprises a window comparator having an input for receiving a filtered, demodulated version of the high-frequency modulated signal, and an input for receiving a level signal which is a measure of the level of the high-frequency modulated signal. The in-channel signal is formed, by means of a combination device, from the output signals of the window comparator combined with the level signal.
The invention also relates to a receiver for receiving high-frequency signals and comprising such a detection device. Such a receiver may be a radio receiver, a television receiver, a video recorder or a cordless or mobile telephone.
2. Description of the Related Art
A detection device of this kind for the detection of a tuning state of a high-frequency modulated signal within a frequency band is known from Japanese Patent Application No. 58-115419, Kokai 60-7222/85. The known detection device, intended to reduce so-called muting in an FM receiver, comprises a window comparator whereto there is applied a filtered, demodulated version of a high-frequency modulated signal to be applied to the FM receiver, and also comprises an input for receiving a level signal which is supplied by a level device which is included in the FM receiver and which serves to produce a level signal which is a measure of the level of the high-frequency modulated signal. The known detection device also comprises a combination device which is formed by two gates whereby an in-channel signal is formed output signals of the window comparator and from the level signal. The in-channel signal is applied to tuning control means which are not shown in the cited Kokai.
It is known that the detection characteristic of a discriminator such as an FM detector is S-shaped in relation to an RF frequency sweep around a frequency whereto the tuner is tuned. This characteristic passes through zero at the tuning frequency, is referred to as the so-called S-curve. A window comparator, consisting of two comparator circuits with hysteresis, imposes two detection thresholds on the S-curve, so that it can be determined whether the filtered demodulated version of the high-frequency modulated signal is around a signal zero level which is between the two detection thresholds. In order to avoid that the detection device provides an in-channel indication when no station is being received, i.e. if there is no RF signal, level information of the RF signal is also taken into account. An AND-combination of the signals derived from the S-curve detection and the level detection produces an in-channel signal. In a search mode, pulses from the detection device can be filtered, if desired, in order to obtain a suitably defined in-channel signal.
A problem encountered during use of the known detection device in inter alia the search mode in an FM receiver, consists in that incorrect in-channel signals may be generated, which incorrect signals may cause tuning to a "wrong" slope of the S-curve where no station is situated; this may occur notably in the event of a frequency band comprising many stations which may be situated very near to one another. This is because the signal level of a station may then be situated between the signal levels of two other stations which are above the adjusted detection thresholds, and because intermediate frequency filters do not adequately suppress the adjacent channels. In practical cases the S-curve will also be distorted in the event of a busy frequency band. Moreover, the requirements imposed on the low-pass filter preceding the detection device will differ in the preset mode and the search mode, and choosing a compromise may give rise to an increased number of mistunings. Measurements have shown that wrong slopes of the S-curve over the frequency band tend to be steeper than correct slopes, so that incorrect in-channel pulses generally are shorter than correct in-channel pulses. When use is made of a filter connected to the output of the detection device, requirements which are more difficult to combine are then also imposed on this filter. The compromise to be chosen in this respect may also lead to an increased number of mistunings.